Bupropion is an antidepressant chemically unrelated to tricyclics, tetracyclics, selective serotonin re-uptake inhibitors (SSRIs), or other known antidepressant agents. The drug resembles a psycho stimulant in terms of its neurochemical and behavioral profiles in vivo, but it does not reliably produce stimulant-like effects in humans at clinically prescribed doses. Its structure closely resembles that of diethylpropion and it is related to phenylethylamines. It is designated as (±)-1-(3-chlorophenyl)-2-[(1,1-dimethylethyl)amino]-1-propanone hydrochloride and by its generic name amfebutamone hydrochloride. Bupropion hydrochloride is commercially available as an immediate release form (Wellbutrin®) and a sustained release form (Wellbutrin® SR and Zyban®). Both Wellbutrin® SR and Zyban® are chemically and pharmaceutically identical.
The neurochemical mechanism of the antidepressant effect of bupropion is not well known. Bupropion does not inhibit monoamine oxidase. Bupropion affects chemicals within the brain that nerves use to send messages to each other. These chemical messengers are called neurotransmitters. The neurotransmitters that are released by nerves are taken up again by the nerves that release them for reuse (this is referred to as reuptake). Many experts believe that depression is caused by an imbalance among the amounts of neurotransmitters that are released. It is believed that bupropion works by inhibiting the reuptake of the neurotransmitters dopamine, serotonin, and norepinephrine, an action which results in more dopamine, serotonin, and norepinephrine made available to transmit messages to other nerves. Accordingly, bupropion is unique in that its major effect is on dopamine, an effect which is not shared by the SSRIs (e.g. paroxetine (Paxil®), fluoxetine (Prozac®), sertraline (Zoloft®)) or the tricyclic antidepressants or TCAs (e.g. amitriptyline (Elavil®), imipramine (Tofranil®), desipramine (Norpramin®)).
Wellbutrin® and Wellbutrin® SR are used for the management of depression. Zyban® has been approved as an aid to patients wanting to quit smoking. Wellbutrin®, the immediate release formulation of bupropion, is dosed three times a day, suitably with 6 or more hours in between doses. For patients requiring more than 300 mg bupropion a day, each dose should not exceed 150 mg. This requires administration of the tablets at least 4 times a day with at least 4 hours in between doses. The immediate release formulation results in more than a 75% release of the bupropion into the dissolution media in 45 minutes, and one of the major side effects of bupropion has been the incidence of seizures, which in part appears to be strongly associated with the immediate release of the bupropion into the system. Accordingly, sustained release products were developed to avoid the incidence of seizures. The sustained release products are dosed twice daily.
In general, patient compliance is a problem with medications that require a multiple dosing regimen and is especially problematic with depressed individuals. While sustained release formulations have simplified the dosing regimen and increased patient compliance, there is still room for further simplifying the dosing regimen and further improving patient adherence to the dosing regimen. The development of an approved stable once daily modified-release bupropion formulation would be an advance in the art.
The selection of a suitable salt for a drug candidate is recognized as an important step in the preclinical phase of drug development; however, the scientific literature on this topic is rather limited. Changing the salt form of a drug is a recognized means of modifying its chemical and biological properties without modifying its structure. As yet, there is no reliable way of predicting exactly what effect changing the salt form of an active drug will have on its biological activity. A decision to change the salt form at a later stage introduces the need to repeat toxicological, formulation and stability tests, with obvious implications for the overall development and production time for the new pharmaceutical product.
In general, a few of the factors that should be considered during a salt selection include: What is the effect of the salt on the chemical stability of the drug substance and the drug product? Does the salt form a hydrate? What is the solubility of the salt and is it appropriate for in vivo administration? What is the quality of the salt with regard to processing, issues with scale up, safety, etc.?
According to the Chemical Abstracts Registry® Database, the only salts of bupropion that have been previously reported are the hydrochloride (HCl), (2Z)-2-butenedioate, (2E)-2-butenedioate, methane sulfonate, formic acid, 2-hydroxy-1,2,3-propanetricarboxylate, phosphate and trifluoromethanesulfonate salts.
There is a need for a once daily formulation of a pharmaceutically acceptable salt of bupropion with enhanced stability.